Since rechargeable batteries are an important part of mobile devices, there has been much research and development in battery systems that allow a high energy density along with a fast charging rate. The following figure shows the currently preeminent secondary battery systems and their energy densities in terms of both weight and volume.

Secondary cell energy density, in terms of both weight and volume. Research helps us to go farther to the upper right hand corner. (Via Wikimedia Commons))

This is not a very good storage battery by today's standard, since it has an energy density of just 50 Wh/kg, far towards the lower left quadrant of the figure. There's the further problem that nickel has become an expensive metal, costing about seven dollars a pound, as compared to the $0.80 per pound cost of lead. However, a century ago, when nickel wasn't too expensive and storage batteries were generally lead-acid, this battery type caught the interest of Thomas Edison.

Edison's nickel-iron batteries had a somewhat higher energy density than lead-acid batteries, and they could be charged twice as fast. The hydroxide electrolyte was less responsive than sulfuric acid at low temperatures; and, because of the nickel content, the batteries were more expensive. However, they were extremely rugged and used in many industrial applications. They did prove suitable for electric vehicles, as a thousand mile endurance test in 1910 proved.[2]

In Edison's battery, the iron electrode was a composite of graphite and iron powder.[3] In the nanoscale version, the nickel oxide-hydroxide and iron exist as nanoparticles that sit on either graphene or carbon nanotubes, so they have a larger area. As a consequence, these novel batteries have a thousand-fold faster charging and discharging rate than their conventional counterparts. they also have a high energy density.[5]

The batteries could be charged in about two minutes, and discharged within thirty seconds. Their energy density of 120 Wh/kg and specific power of 15 kW/kg put them at the same performance level as other rechargeable batteries, and much higher than conventional nickel-iron batteries.[5] Unlike its lithium-ion battery competitors, this battery uses a non-flammable electrolyte of water and potassium hydroxide.[3]

In theory, this new battery has the same per weight energy density as the lithium-ion battery in the Nissan Leaf, an all-electric car.[4] There's a potential for cost savings, since iron and nickel are more abundant than lithium, and thereby less expensive. The caveats are that the lab-scale cell has only powered a flashlight,[3] and the capacity is reduced by 20% after 800 discharge/charge cycles.[3]